This invention relates to a process for the removal of benzene from petroleum fuels and gasoline. More particularly, this invention relates to a process for the controlled hydrogenation benzene utilizing a light naphtha isomerization process.
Largely paraffinic crude naphtha fractions, generally comprising C.sub.4 to C.sub.12 hydrocarbons and having a boiling point generally below 425.degree. F. at atmospheric conditions, are a significant source of gasoline pool blending components. As produced from a refinery crude unit or pipestill, these paraffinic crude naphtha fractions have a low octane rating and without upgrading or blending with a higher octane stream, can comprise only a small fraction of the finished gasoline pool. Historically, the addition of lead was useful for upgrading gasoline pool octane, permitting the blending of higher amounts of low octane paraffinic crude naphtha directly into finished gasoline. However, lead phase-out eliminated this as a cost-effective alternative to enhance gasoline pool octane.
Refiners now commonly utilize downstream processing steps to enhance paraffinic crude naphtha octane. Paraffinic naphtha fractions comprising C.sub.7 to C.sub.12 hydrocarbons are generally reformed into higher octane aromatics by a combination of dehydrogenation and dehydrocyclization. The reformate product produced from such reforming processes, comprising aromatics such as benzene, toluene, and xylene, can also be separated from the gasoline pool and marketed as feedstock for chemical manufacture or sold in component form to other refiners as octane support. The reforming process also manufactures hydrogen as a by-product, which has become particularly useful to refiners who now must meet ever decreasing fuels sulfur level targets.
Paraffinic naphtha fractions comprising C.sub.4 to C.sub.6 hydrocarbons, and more particularly comprising C.sub.5 to C.sub.6 hydrocarbons (light paraffinic naphtha), are generally isomerized in an isomerization process to higher octane branched isoparaffins. Isomerization is generally preferred to reforming for paraffinic C.sub.5 hydrocarbons since such hydrocarbons are not easily reformed into aromatics. Isomerization of paraffinic C.sub.6 hydrocarbons is generally preferred to reforming since the reforming of such hydrocarbon results in a lower reformate volume yield than typical of reformate produced from C.sub.7 to C.sub.12 hydrocarbons, wherein the octane benefits derived from reforming are outweighed by the loss of gasoline volume yield.
The isomerization of C.sub.5 to C.sub.6 paraffinic naphtha fractions is generally believed to be a first-order reversible reaction that is constrained by thermodynamic equilibrium between the normal paraffinic feedstock and the various isomers of the feedstock. Most isomerization processes are categorized as either low-temperature or high-temperature isomerization. Low-temperature isomerization processes generally utilize a highly-chlorided platinum on alumina catalyst which provides high catalyst activity and permits operation at lower temperatures. High-temperature isomerization processes typically utilize a catalyst containing platinum or other noble metals on a molecular sieve-based support which provides lower catalyst activity and necessitates operation at higher temperatures. Both processes utilize catalysts of sufficient activity to generate undesirable side-reactions such as disproportionation and cracking. These side-reactions combined with paraffinic naphtha dehydrogenation, not only decrease the product yield but can form olefinic components that increase catalyst deactivation. These undesired reactions are generally controlled by carrying out the isomerization reaction in the presence of a hydrogen-containing stream.
New clean air legislation, which substantially reduces the allowable benzene content in gasoline, may necessitate significant changes in the management of C.sub.4 to C.sub.12 paraffinic naphtha fractions. Although refiners can target reformer feed streams to contain substantially C.sub.7 to C.sub.12 components, modern fractionation processes generally result in some percentage of paraffinic C.sub.6 hydrocarbon remaining in the reformer feed, wherein such hydrocarbon is subsequently reformed to benzene and other reaction products. Where fractionation steps are adjusted to ensure only minimal paraffinic C.sub.6 hydrocarbons enter the reformer feed, such as by increasing the temperature cut point between isomerization unit feed and reformer feed, additional volumes of C.sub.7 hydrocarbons are fractionated into isomerization unit feed. Hydrocarbons having 7 carbons or more are known to substantially increase the deactivation rate of modern isomerization unit catalysts and are more susceptible to hydrocracking, resulting in a reduction in liquid yield and high reaction temperature exotherms. Moreover, some C.sub.6 hydrocarbons are formed from the hydrocracking of higher boiling hydrocarbon in the reforming process and subsequently reformed to benzene. Such benzene production cannot be avoided by improved fractionation techniques. Additionally, benzene has a relatively high octane number, and processing steps to destroy or limit production of any high octane component, may necessitate the replacement of the lost octane through more costly processing alternatives.
Therefore, there is a great need in the petroleum refining industry, for a cost effective, safe, and operationally controllable method for reducing the benzene content in gasoline. Several methods have been suggested to address the aforementioned need, each meeting with varying degrees of success.
Saturation of benzene by processing a benzene-containing stream directly over an isomerization catalyst, has been utilized to reduce benzene concentrations where small concentrations of benzene are involved. This process generally requires fractionation of C.sub.5 to C.sub.6 paraffinic naphtha streams derived from crude, in a manner so as to include a substantial portion of the C.sub.5 and C.sub.6 cyclics (including benzene) in the isomerization unit feed stream. The isomerization unit feed stream can also be supplemented with C.sub.6 cyclics manufactured in other refining facilities, including but not limited to catalytic reforming processes, and processed directly into an isomerization unit. With the hydrogen that is present in the isomerization unit reaction zone and the high activity of the isomerization catalyst, substantially all of the benzene is quickly hydrogenated.
U.S. Pat. No. 4,834,866 to Schmidt teaches such a conversion process for feedstocks comprising C.sub.4 hydrocarbon to hydrocarbon boiling at a temperature of about 400.degree. F. (C.sub.12) which includes a fractionating step upstream of the isomerization reactors for separating the feedstock into an overhead stream comprising methyl pentane and hydrocarbon boiling at a temperature lower than methyl pentane, a side cut stream comprising hydrocarbon boiling at a temperature higher than methyl pentane and lower than cyclohexane or benzene, and a bottoms stream comprising hydrocarbon boiling at a temperature higher than cyclohexane or benzene. The side cut fraction is sent to an isomerization zone where the normal hexane is isomerized and substantially all of the benzene saturated. The product of the isomerization reactors is recycled back to the fractionating step where the isoparaffins can be removed to the overhead stream and the unconverted materials, including any unconverted benzene, recycled to extinction through the side cut fraction.
While benzene saturation processes, utilizing a paraffinic naphtha isomerization zone, are particularly effective for converting benzene to an environmentally more acceptable form, there are substantial processing penalties. Cyclic hydrocarbons present or formed in an isomerization unit reaction zone are generally adsorbed into the isomerization catalyst. Adsorption of cyclic compounds on the active sites of the catalyst generally inhibits normal paraffin isomerization resulting in a reduction in conversion of normal paraffins to their respective isomers and a reduction in isomerate product octane.
Apart from the conversion inefficiencies caused by processing C.sub.5 to C.sub.6 cyclics in an isomerization reaction zone, the saturation of benzene is highly exothermic. The saturation of benzene in a feedstream generally results in a feedstream temperature increase or exotherm across the isomerization reaction zone of about 20.degree. F. per weight percent benzene in the feedstream. For purposes of the present invention, reaction zone exotherm shall be defined as the temperature difference between the reactor outlet temperature at a location where the exit pipe leaves the reactor (reactor outlet) and the reactor inlet temperature at a location where the inlet pipe enters the reactor (reactor inlet), for the reactor within the reaction zone having the largest reaction zone exotherm. High isomerization reaction temperatures are generally unfavorable since they inhibit the formation of more desirable higher octane doubly branched isomers and result in lower yields of high octane isomers such as 2,2-dimethylbutane (J. A. Ridgeway, Jr. and W. Schoen, ACS Symposium, Div. of Petroleum Chemistry, Boston, Apr. 5-10, 1959, A-5-A-11). Higher reaction zone temperatures similarly increase the rate of carbon laydown (coking) on the catalyst resulting in catalyst deactivation.
It is also known that benzene can be converted to more environmentally acceptable forms through hydrogenation in a separate reaction step upstream of an isomerization zone.
U.S. Pat. No. 5,003,118 to Low et al. teaches a process for hydrogenation and decyclization of benzene which requires passing an isomerization unit feedstream, including all of the benzene, over a hydrogenation catalyst comprising either a platinum group metal and tin on a solid support or a platinum group metal and cobalt and molybdenum on a solid support. The hydrogenated product is then directed, without additional heat input, to an isomerization zone where the hydrogenated product is isomerized and an isomerate product produced. The Low et al. process utilizes the heat derived from saturating benzene as the entire heat source for obtaining the isomerization zone operating temperature.
While the Low et al. process effectively mitigates the penalties associated with benzene adsorption on the isomerization reaction zone catalyst, other cyclics which are derived from the saturation of benzene, such as cyclohexane and methylcyclopentane, are still directed to the isomerization reaction zone resulting in less effective isomerization. Moreover, there are several drawbacks to processes which rely on a benzene hydrogenation exotherm for providing optimum isomerization reaction zone temperatures. Isomerization facilities generally process a feedstock fractionated directly from petroleum crude. The benzene concentration of the petroleum crude-derived light paraffinic naphtha can vary substantially, causing wide variations in the benzene hydrogenation exotherm. Where there is no supplemental or controlling heating or cooling source between the hydrogenation and isomerization reaction zones, the isomerization reaction zone temperature cannot be regulated optimally or even reliably.
Large swings, and particularly upward excursions in the hydrogenation reaction zone temperature can also cause hydrocracking to occur in the hydrogenation or isomerization reaction zones and a subsequent hydrogenation or isomerization reaction zone temperature runaway condition. Higher hydrogenation or isomerization reaction zone temperatures increase hydrocracking, which increases the yield of undesirable lighter hydrocarbons at the expense of liquid products. It is important to note that hydrocracking is also a highly exothermic reaction. Deviations in the concentration of benzene in isomerization unit feedstreams can cause significant hydrocracking to occur in modern isomerization units, compounding the benzene saturation exotherm with a hydrocracking exotherm, and resulting in a temperature runaway condition. A temperature runaway scenario generally deactivates the isomerization catalyst resulting in severe process penalties, and under more extreme situations, can result in damage to equipment, potential unit shutdown, and can present a safety hazard to facility personnel.
Aside from the cost penalties associated with loss of optimum process control, processes similar to those described above are limited as to the capacity of benzene processed. Since benzene hydrogenation capacity is generally exotherm limited to a particular concentration of benzene in the feedstock, benzene that is naturally occurring in the crude-derived paraffinic naphtha feedstock limits the volume of supplemental benzene (at a higher benzene concentration) that can be added from other refinery processes before reaching the exotherm limit. Therefore, processes utilizing a separate benzene hydrogenation zone upstream of an isomerization zone introduce a new array of process penalties and operability problems that must be eliminated or solved.
It has now been found that the addition of a benzene hydrogenation zone downstream of the isomerization zone of an isomerization unit combined with the addition of supplemental refinery benzene-containing streams downstream of the isomerization zone and upstream of a hydrogenation zone, provides superior operability and process economics to the prior art processes. Process penalties incurred from the adsorption of benzene on the isomerization catalyst active sites through the addition of supplemental streams containing cyclics such as benzene are eliminated since supplemental benzene is added downstream of the isomerization zone. Benzene hydrogenation exotherms, temperature swings, and excessive hydrocracking caused by supplemental benzene sources generally do not adversely affect the isomerization reaction since hydrogenation of the benzene from the supplemental benzene sources occurs downstream of the isomerization reaction. Benzene processing capacity, at constant and controllable benzene hydrogenation exotherms, is also increased since the isomerization reaction zone product contains minimal benzene and provides a larger heat sink for absorbing the temperature exotherms created from the hydrogenation of benzene from supplemental sources.
It is therefore an object of the present invention to provide a process for the hydrogenation of benzene and the isomerization of a light naphtha feedstock that substantially reduces the benzene concentration of crude paraffinic naphtha fractions and supplemental high benzene-content streams processed in an isomerization facility.
It is another object of the present invention to provide a process for the hydrogenation of benzene and the isomerization of a light naphtha feedstock that achieves substantial benzene reduction wherein the benzene from supplemental benzene-containing streams does not significantly affect isomerization conversion or deactivate the isomerization catalyst.
It is another object of the present invention to provide a process for the hydrogenation of benzene and the isomerization of a light naphtha feedstock wherein exotherms created from the hydrogenation of benzene are controllable and do not create unstable exotherms that can cause operability problems, excessive hydrocracking, and temperature runaway conditions.
It is yet another object of the present invention to provide a process for the hydrogenation of benzene and the isomerization of a light naphtha feedstock that can process higher supplemental benzene-containing stream benzene concentrations and volumes at a constant and controllable exotherm temperature than prior art processes.
Other objects appear herein.